Caveats

Text is spoken directly to the object or avatar within the same region as the script.

Scripts in tasks other than target can not listen for and receive these text messages, with an exception for attachments as described below.

Text can be a maximum of 1024 bytes. This can convey 1024 ASCII-7 characters, or 512 UTF-8 characters such as á

A prim cannot hear itself, to prevent problems with recursion.

Sending text on DEBUG_CHANNEL is not supported

Text sent to an avatar's ID on channel zero will be sent to the receiver's viewer.

Text sent to an avatar's ID on non-zero channels can be heard by any attachment on the avatar

A quick series of messages sent by llRegionSayTo cannot be relied on to arrive at their destination in the same order as sent.

There is a per-destination throttle of 200 messages sent over 10 seconds on channel zero. Thereafter, there is also a per-sending-object throttle of 100 messages on channel zero over 300 seconds (5 minutes). This throttle is relevant only after the first throttle is activated. Messages sent on channels other than zero do not trigger the throttle but, once it is triggered, the block applies to all channels. The block is region-wide and applies to all objects with the same owner. The block apparently lasts until the region is restarted.

Negative channels are popular for script communications because the standard Second Life client is unable to chat directly on those channels ("/-xxxx message" won't chat "message" on channel "-xxxx", it will chat "/-xxxx message" on channel zero).

Messages sent on channel zero via LLRegionSayTo() are blocked after 200 messages on to the same destination in a 10 second period. After this throttle is activated, there is a further throttle, on objects belonging to the owner of the object that triggered the first throttle, of 100 messages per object over 5 minutes (see https://jira.secondlife.com/browse/BUG-5457). The block is region-wide and applies to all objects belonging to the owner, so if a script triggers the throttle, all objects in the same region belonging to the same owner will also be blocked. Messages sent on channels other than zero do not trigger the throttles. The throttles apparently reset only when the region is restarted.

Therefore, llRegionSayTo() is unsuitable for (say) verbose script debugging, and even moderate use can break users' content completely unrelated to the script for an indeterminate (and potentially very long) period of time. Consider using llOwnerSay() for debugging instead or, if this not appropriate, try llRegionSayTo(avatar, non-zero-channel, message) and have an attachment relay the message using llOwnerSay().